Soil is a mixture of weathered rocks, air, water, organic matter and living organisms.

Importance of Soil:

  • It is the natural mediun which holds plant roots and where seeds are put to germinate.
  • Soil supplies plants with mineral nutrients needed for growth.
  • Soil provides water to plants. Nutrients are dissolved in water before they are taken up by the plants.Water keeps the plants color on bat days through a process called transpiration.
  • Soil provides mechanic support or anchorage to the plant.
  • Soil habours micro organisms and also provides them with warmth, air and water.


Soil formed from mixture of wet hored material  from crops and decompose plant and animal material.

Factors that influence soil formation.

  • Parent rock
  • Climate
  • Topography
  • Biotic factor
  • Time


This is the original rock from which soil is formed. Parent rocks differ in structure, composition and the rate of decomposition. The nature of the rock will determine the physical and the chemical properties of the soil that will result e.g.  if the rock has a lot of granite it will give use to course textured acidic sandy soils.



Different agents of after have physical and chemical effects on breaking down of rocks – a process known as weathering.

Weathering; is the process in soil formation.

  • Rain fall

It has both physical and chemical effects o the rock that will cause it break down.

Physical effects

  • Direct impact of rain drops on rocks helped to dislodge small rock particles that are later weathered to form soil.
  • As surface run off moves carrying rock particles, they hit against each other and become smaller.
  • During and after rain the amount of water in the streams and rivers increase, the speed with which the water moves will also increase so it will nit the river bank and cause some rock particles to come off.

Chemical effects

  • Rain water can dissolve carbon dioxide from the atmosphere to form a weak acid known as carbonic acid , this will dissolve away part of the rock and causes it to disintegrate.
  • Most chemical reaction are faster in the presence of water so rainfall will increase the rate of chemical weathering.
  • Soluble substances can easily be leached out of the soil living acidic environment.


In semi-arid and arid areas there is a big different between day and night temperatures. During the day when it is very hot rocks expand and at night it gets very cold and the outside rock will contract very first before the side rock contracts.  There is repeated many times will lead to cracking of the rock and after along time, layers of the rock will peel off a process known as exfoliation.

Chemical effects

  • At higher temperatures all chemical weathering processes will occur at a higher rate. Activities of soil organisms that break down organic matter to form part of soil will be faster at a higher temperature up to a particular point.
  • In places that experience high temperatures, there is rapid evaporation of water from the surface and this leads to accumulation of salts just below the soil surface.


In deserts where there is very little vegetation, there are strong winds and they carry sand particles, when these winds hit a stationary rock they will cause some rock particles to come off so that after along time a heap of particles will be seen at the base of the rock.

temperature soils


Is the nature of physical set up of land. It affects soil formation thorough

  • having an effect on climate
  • having an effect on water movement

Effect of topography on climate and therefore on soil

  • In high areas where it is cool, the rate of chemical weathering is slower
  • The aspect of the soil will determine exposure of the soil surface to sunlight, wind and rain fall. Some places are in the rain shadow so weathering will be slower.
  • Topography affects local rainfall distribution e.g. some places receive relief rainfall and so chemical weathering will be higher in such places.

Effects of topography on water movement and therefore on soil formation

  • On steep slopes, soil particles are removed by erosion so they tend to have shallow soils.
  • What is removed from the steep slopes is laid down in valley so they tend to have deep fertile soils.
  • Steep slops are well aerated so there will be oxidation for example of iron compounds in the rocks leading to reddish soils.

Biotic Factors

This includes all living things and their products.

Physical effects

  • Plant roots grow in cracks in rocks and they cause the cracks to widen. This repeated many times will cause part of the rock to break off.
  • Burrowing animals can break down softer rocks as they move.
  • Organisms that live in soil mix organic matter with rock particles to form soil of a crumbs structure.
  • Animals with hooves break the rock surface over which they move into smaller particles.
  • Man’s activities for example mining, road construction breakdown rocks.
  • Vegetation cover reduces the rate at which soil particles are moved from place to place – erosion.
  • Grass and other plants with the fibrous root system will hold soil particles together so that they are not damaged by rain or wind action.

Chemical effects

  • All living things die and decay as a result of bio-chemical reactions to form organic matter which is part of soil.
  • Micro organisms break down organic matter and in the process organic acids are formed these will be able to break some of the rocks.
  • Lochons, Argo and fungi are known as primary colonizers of rocks. They keep on dying and decaying so that with time a thick layer of organic matter is formed on the rocks. This makes the rock environment better so that higher plants can now grow on the rocks e.g. ferns.
  • Tips of plan roots produce chemical substances that can dissolve away part of the rock therefore weakening it.
  • Burrowing animals make tunnels that improve on soil aeration and drainage and this will increase the rate of chemical weathering.
  • Earth worms produce a cast which is rich in nitrogen therefore can improve on soil fertility. The cast is also rich in calcium carbonate and this will reduce on soil acidity. It has mucus / slimy so it leads to formation of a crumb structure.


Soil formation takes a very longtime and it is a continuous process. The more mature a soil is the better it is for crop growing.


ASSIGNMENT : SOILS IN AGRICULTURE assignment MARKS : 10  DURATION : 1 week, 3 days


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